An oil-producing well may produce hydrocarbons or a combination of hydrocarbons and water from a hydrocarbon reservoir. Factors that may determine whether pure hydrocarbons or a mixture of hydrocarbons are extracted from the reservoir rock are the porosity of the reservoir rock and the water saturation at the depth where fluid is extracted from the reservoir. Reservoir rock may retain an amount of water that may not be produced. This water, although present, is held in place by capillary forces and will not flow. The irreducible water saturation, Swirr, characterizes this amount of water as a ratio between the volume of non-producible water held by the capillaries of the reservoir rock and the total porous volume of the reservoir rock. If the actual water saturation at the extraction site is above the irreducible water saturation, Swirr, a mixture of hydrocarbons and water may be produced, whereas only hydrocarbons may be produced if the actual water saturation is at Swirr. Accordingly, in the oil industry, the irreducible water saturation, Swirr, is a useful parameter for predicting the productivity of a hydrocarbon-bearing reservoir.
Different methods for estimating Swirr exist. Swirr may be estimated from log analysis methods such as, for example, resistivity well logs and nuclear magnetic resonance (NMR) analysis. The industry standard procedure has been to validate these results using saturation height modeling based on special core analysis (SCAL), where the irreducible water saturation, Swirr, of a core sample is determined in the lab. Using the lab-determined irreducible water saturation, Swirr, saturation height modeling can be applied to predict the actual water saturation in the well, depending on the height above the free-water level in the well, where the water saturation reaches 100%. Multiple methods for performing Swirr analysis on a sample core exist. Commonly used methods include, for example, the Mercury Injection Capillary Pressure (MICP) method and oil-brine and air-brine centrifugation drainage and porous plate methods.